The present invention relates, generally, to systems and methods for magnetic resonance imaging (MRI) and, more particularly, to systems and methods for generating images based on patient-specific morphological data and images.
Femoroacetabular impingement (FAI) is a common cause of intra-articular hip pain resulting in labral tears and associated chondral lesions, which are precursors to hip osteoarthritis (OA). Improved understanding of the condition has led to treatment strategies that seek to both correct these abnormalities and repair the intra-articular damage they have caused. Current trend in orthopedic surgery is focusing on joint preservation instead of joint replacement. Recently many pre-arthritic conditions based on subtle anatomic abnormalities in a young population have been identified. Improved understanding of these conditions has led to treatment strategies that seek to both correct these abnormalities and repair the intra-articular damage they have caused before frank osteoarthritis and therefore irreversible damage will occur. The most important predictor for treatment success is reported to be the integrity of the articular cartilage surfaces. Moderate cartilage damage, unfortunately, can be a challenge to diagnose. Radiographic evaluation using Tönnis grading is the standard of care but has been shown to have poor interobserver reliability. Increasingly, magnetic resonance imaging has been used to investigate these structures for such purposes.
When a substance, such as human tissue, is subjected to a sufficiently-large, uniform magnetic field (polarizing field B0), the individual magnetic moments of the nuclei in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to another magnetic field (excitation field B1) that is in the x-y plane and that is near the Larmor frequency, the net aligned moment, Mz, may be rotated, or “tipped”, into the x-y plane to produce a net transverse magnetic moment Mxy. A signal is emitted by the excited nuclei or “spins”, after the excitation field B1 is terminated, and this signal may be received and processed to form an image.
When utilizing these “MR” signals to produce images, magnetic field gradients (Gx, Gy, and Gz) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received MR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
Although clinical MR imaging has evolved as a reliable tool in depicting labral tears, the adequacy of MRI for cartilage assessment remains poor. Identifying cartilage damage in FAI may be difficult due to the pattern of cartilage damage particular to this condition. In FAI, cartilage damage is frequently limited to the acetabulum and occurs deep within the tissue as a debonding of articular cartilage from bone. This leaves the superficial layer intact, a pattern uniquely ill suited for diagnosis with traditional MRI, which is best at detecting a void at the articular surface. Thus, assessing articular cartilage with MRI requires both high resolution and high contrast-to-noise ratio, especially in the hip. With cartilage quality being the primary prognostic information for any type of joint preservation surgery, and given the difficulty faced today in its imaging and classification, patients inappropriate for joint preservation are not always identified and inappropriate treatments are sometimes recommended.
New MR quantitative cartilage imaging techniques have the potential to be diagnostic and therefore improve treatment decision-making. However, even though arthroscopy is the only practical gold standard (biopsy for histology of patient cartilage is contra-indicated), there are many limitations due to poor spatial correlation of MR findings obtained in slices and the arthroscopy viewed in 3-D.
For example, some have turned to quantitative MR mapping techniques, such as delayed gadolinium-enhanced MRI of cartilage (dGEMRIC). dGEMRIC is the most widely applied investigational technique. It can, however, be time-consuming, logistically difficult to perform, and currently gives a combined value for femoral and acetabular cartilage. As a further limitation of the technique, there is a patient population that cannot be subjected to a dose of gadolinium, such as those with decreased renal function or a history that otherwise implicates the kidney.
Therefore, it would be desirable to have a system and method for visualizing joint and similar structures in a subject as a mechanism to investigate such structures for a variety of purposes.